Giant planets may host superionic water

Chemists have recreated the conditions inside the giant planets with which we share our Solar System. And they've shown that the water inside giants such as Neptune might act very strangely indeed.

The conditions in such planets are extreme: heat of more than 1,000 ºC and pressure some 100,000 times what we are used to. Ordinary substances could behave in very strange ways inside these scalding behemoths. And that includes water, as both computer modelling and actual experimental studies now show.

“It is best to think of it as a lattice of oxygens which are basically fixed, but the hydrogen atoms are free to move.”

Laurence FriedLawrence Livermore National Laboratory

It has been predicted for some years that water under such conditions would act neither as a straightforward solid or liquid but exist in a 'superionic' phase, in which the oxygen atoms are essentially frozen, but the hydrogen atoms can whiz around at high speed1.

Laurence Fried and his colleagues at Lawrence Livermore National Laboratory in California decided to see if they could get water to go superionic. To create the immense pressures they needed, the team used a device that smashed water between two diamonds. They then heated the water with an infrared laser beam.

As they did so, the researchers monitored the frequency with which the water molecules vibrated, and looked out for an abrupt shifts in frequency that would signal that the water had altered its state, or 'phase'. "By looking at that we are able to determine phase boundaries, but we don't really know what is on the other side of the boundary," says Fried.

Lattice work

The researchers also studied computer models of the atoms' behaviour, which suggested that the water had indeed entered a superionic phase, a strange state between solid and liquid. Tracking a group of some 60 simulated atoms took weeks, and required computing power equivalent to 1,000 laptops.

The model showed that as temperature and pressure increase, the molecules break apart, settling into a non-molecular pattern that is denser than normal ice. Beyond that, it shifts to the superionic. It's hard to imagine, Fried admits. "It is best to think of it as a lattice of oxygens that are basically fixed, but the hydrogen atoms are free to move," he explains.

If you brought such water into a regular room on Earth, it would explode, or "go poof", as Fried puts it. But inside a planet it would be hard as iron and so hot that it would glow bright yellow. Fried presented the work at the American Chemical Society's meeting last week in San Diego, California.

"I think its very nice work, and the fact that they've done calculations as well as experimental work makes it a very credible story," says Russell Hemley, who studies high-pressure chemistry at the Carnegie Institution's Geophysical Laboratory in Washington DC. He adds that he would like to see more direct evidence of the existence of superionic water in the lab. Fried agrees, and is aiming to measure how fast heat travels through the substance.

If superionic water really does exist in the hearts of giant gas planets, speculates Fried, there might be more of it in the Solar System than there is of more familiar types of water. What's more, he adds, its potentially excellent electrical conductivity might account for the huge magnetic fields of planets such as Neptune and Uranus.